The field of the present invention relates to mechanical joints incorporating braking mechanisms. In particular, a mechanical joint is described producing varying braking torque in response to motion of the joint.
A mechanical joint assembly is an assembly which allows two members to be mechanically joined but allows rotation of one member with respect to the other about one or more rotation axes. A universal joint is a mechanical joint assembly providing rotation about two substantially orthogonal axes. Such a joint is useful in applications in which two members must be mechanically joined but must be allowed to assume an arbitrary relative angle.
In some applications it is desirable for the motion of the moving members about the universal joint to be restricted or damped by braking. For example, such a braked universal joint is useful in situations wherein a first joined member is a boom with a second joined member suspended therefrom being some load carrying means, wherein motion of the load carrying means must be restricted, particularly when not loaded. One particular application of a braked universal joint is suspension of a grappler from a boom of a logging skidder. Several previous designs for a braked universal joint used in this way (also referred to as a swivel link) are described in U.S. Pat. Nos. 4,335,914; 4,417,759; 4,573,728; 4,679,839; 4,715,641; 5,110,169; 5,451,087; and 5,601,161, each of said patents being incorporated by reference as if fully set forth herein. A much improved design for a swivel link incorporating frusto-conical surfaces for braking and load-bearing is disclosed in U.S. Pat. Nos. 5,713,688 and 5,779,383 issued to the applicant of the present application, both of said patents being incorporated by reference as if fully set forth herein. The design and construction of swivel link assemblies, many drawbacks of previous swivel link designs, and improvements resulting from the use of frusto-conical braking and load-bearing surfaces are fully at disclosed therein, and need not be reiterated herein.
A primary figure-of-merit for a swivel link is the number of hours of use in the field before replacement of the friction members of the joint (friction discs in older designs, friction cones in the frusto-conical design). Anything that reduces wear of the friction members (and therefore reduces concomitant down time, maintenance time, and maintenance costs) is highly desirable.
During use of a swivel link, and many other braked mechanical joints, it is often the case that small motions about the joint are insignificant and can be tolerated, while larger motions are undesirable and must be prevented (for example, to prevent injury or equipment damage). However, previous joints provide a constant braking torque independent of the amount of motion which has occurred. Reduction of the braking torque for small motions and/or motions near a rest position would reduce unnecessary wear on the friction members, xe2x80x9csavingxe2x80x9d the friction members for suppression of larger amplitude motions and/or motions far from the rest position.
It is therefore desirable to provide a braked mechanical joint assembly in which a relatively smaller braking torque is applied during small amplitude motions and/or motions near a rest position of the joint, while a relatively larger braking torque is applied during large amplitude motions and/or motions far from the rest position of the joint. It may also be desirable to provide a braked mechanical joint assembly in which the braking torque may vary with joint position and/or motion in an arbitrary fashion.
Certain aspects of the present invention may overcome one or more aforementioned drawbacks of the previous art and/or advance the state-of-the-art of braked mechanical joint assemblies, and in addition may meet one or more of the following objects:
To provide a braked mechanical joint assembly wherein the braking torque varies with motion of the joint;
To provide a braked mechanical joint assembly wherein the braking torque increases as the joint moves farther from its rest position;
To provide a braked mechanical joint assembly wherein the braking torque decreases as the joint moves closer to its rest position;
To provide a braked mechanical joint assembly which allows small amplitude motion about a resting joint position;
To provide a braked mechanical joint assembly wherein the braking torque is reduced after a motion is arrested by the larger braking torque produced by that motion;
To provide a braked mechanical joint assembly wherein the wear of friction members is reduced;
To provide a braked mechanical joint assembly wherein the wear of friction members is reduced during motion of the joint near its rest position;
To provide a braked mechanical joint assembly with a longer useful field life than previous joint assemblies; and
To provide a braked mechanical joint assembly with reduced maintenance requirements.
One or more of the foregoing objects may be achieved in the present invention by a braked mechanical joint assembly comprising: a) a first brake member; b) a second brake member rotatable relative to the first brake member; c) a friction member positioned between the first and second brake members; (d) means for pushing the first and second brake members together, thereby frictionally engaging them with the friction member so as to oppose relative rotation of the brake members. The friction member may be integral to one or both of the brake members. A rest braking torque is set at the resting position of the joint by initial adjustment of the pushing means.
A larger braking torque is produced at other joint positions by providing the joint assembly with a bearing assembly for varying the braking torque with movement of the joint. The bearing assembly may comprise mating bearing grooves or raceways provided on pushing means and at least one of the brake members. The depth of each groove varies over its course, having a deepest segment corresponding to each of several bearings (ball or roller bearings) in the groove. When the joint is in its resting position, a bearing is received in each of these deepest segments, the pushing force exerted by the pushing means is at its minimum (i.e., rest position) value, and the braking torque is at its minimum value. As the joint moves away from its resting position, relative rotation of the grooves of the brake member and pushing means causes the bearings to roll out of the deepest segments of the grooves and into shallower segments of the grooves. As the bearings roll into shallower portions of the groove, the cam action of the bearings in the grooves results in increasing pushing force exerted by the pushing means and therefore increased braking torque. By tailoring the depth profile of the groove as a function of joint position, any desired pushing force/brake torque vs. joint position behavior can be achieved.
Additional objects and advantages of the present invention may become apparent upon referring to the preferred and alternative embodiments of the present invention as illustrated in the drawings and described in the following written description and/or claims.